DE69029481T2 - Exposure device - Google Patents

Description

The present invention relates generally to exposure systems used in illuminating original documents for copying.

A number of copying devices are known in which an original document is illuminated and the illumination is directed onto a photosensitive medium to form an image. One such device is shown in US-A-4525059. A variety of photosensitive surfaces may be used, such as a charged photosensitive drum in the case of conventional xerography. As further examples, other media may be used, such as conventional photographic film or a microencapsulated photosensitive medium, such as disclosed in US Patent Nos. 4,440,846 and 4,399,209.

A common method of illuminating the original document is to scan the document with a slit-shaped illumination light. In a typical prior art arrangement, a linear light source mounted within a parabolic cylinder reflector directs a slit of light onto the surface within which the original document is carried. The light is reflected from the original document and collected by a lens or other focusing means and then directed onto the photosensitive medium. The light source and focusing system can be moved together past the original document to scan the document, or the original document can be moved past the light source and focusing system.

With such a copying device, it is advantageous to be able to regulate the contrast level in the output image. While strong contrast is desirable for images with text or line drawings, such a strong Contrast when copying halftone originals to low quality reproductions. Since a particular copying machine is expected to reproduce original documents of both types, the ability to adjust the contrast would greatly improve the performance of the machine.

It is known that contrast can be reduced by using a "burst" exposure. Such an exposure consists of a non-image-related total exposure of the photosensitive medium. This exposure has the effect of reducing or attenuating the contrast when the exposure has occurred. The burst exposure can be performed before, after, or simultaneously with the image exposure.

It is desirable to be able to adjust the amount of burst exposure to control contrast. With a fixed burst exposure, the image to be reproduced may have unacceptably low densities in areas of the image such as text or line art.

Controlled burst exposure can be achieved by means such as an auxiliary burst light source or by performing burst and image exposure separately but using the same light source. However, these techniques have disadvantages in that the control systems required increase the cost and complexity of the copying device. In the case of an auxiliary lamp, the additional cost and complexity are increased by the additional light source. Performing burst and image exposure separately increases the amount of time required to produce an image. In both cases, the power requirements of the device increase.

Therefore, a conventional means is needed to apply a controlled burst exposure to regulate the contrast of an image to be reproduced. Such a means should be easy to control and yet add relatively little to the cost and complexity of the reproductive device.

The present invention arose from our efforts to meet the requirements just mentioned.

In accordance with the present invention, there is provided an exposure apparatus for original documents comprising

- a support (12) for carrying the original (14);

- a slit illumination system (10) for illuminating a slit surface on the original document, which is carried by the support and which comprises a light source (16) which, in operation, directs illumination light onto the carried original document, and further comprises a light directing device (20) for illuminating reflected illumination light onto a light-sensitive medium (22) which the light directing device has received from the field of view of the original document (14) visible to it, the field of view comprising the slit surface,

- a movement device (24) which, during operation, causes a relative movement between the support (12) and the slit illumination system (10) in order to scan the original document (14) with the illumination light and the field of view,

- and a reflector (28), characterized by an adjustable reflector positioning device (32, 34, 38) attached to the slit illumination system for positioning the reflector (28) between the support (12) and the slit illumination system (10) is capable of being adjusted to a position suitable for blocking a portion of the field of view and adjusting the relative proportion of the field of view with respect to the slit illumination system and maintaining such relative proportion during scanning of the original document with illuminating light until further adjustment is made, so that the reflector (28) is adjusted in use to receive a portion of the illuminating light from the light source (16) before it reaches the original document (14), and so the reflector reflects the portion of the illuminating light diffusely to the field of view of the light directing device (20).

As we will explain below, with practical embodiments of such an apparatus we can provide a system for reproducing documents in which the contrast in the image to be reproduced is controlled by the use of burst exposure. The control of the burst exposure is achieved simply and inexpensively.

The invention is described in more detail below by way of example only with reference to the accompanying drawings, in which:

Fig. 1 is a schematic view of an embodiment of the original document exposure apparatus constructed in accordance with the present invention;

Fig. 2 is a partially schematic plan view of an arrangement with mounting and adjustment elements for the diffusion reflector of the device in Fig. 1; and

Fig. 3 is a diagram showing the optical density (D) in the image to be reproduced as a function of the logarithm of the exposure energy used (log E) to illustrate the operation of a device in accordance with the present invention.

An embodiment of the exposure system in accordance with the present invention is shown in Figure 1. A scanning exposure system (10) is mounted beneath a glass plate (12) upon which an original document is placed face down. The scanning system (10) includes one or more (two are shown) illumination light sources (16). These linear light sources are mounted within parabolic cylinder reflectors (18) which direct the illumination light onto the underside of the glass plate (12) where a slit-shaped area of the original document (14) is illuminated. The illumination light is directed into a focusing system (20) which focuses the slit illumination area onto a photosensitive area (22). A corresponding drive mechanism (24) is coupled to the scanning system (10) so that the entire scanning system (10) can be moved as shown by the arrows (26) in a manner well known in the art so that the slit-shaped exposure area can be moved across the entire original document (14).

Lamps (16) of known type are suitable for the photosensitive surface (22) to be exposed. Such a surface may be a charged photosensitive plate as in a conventional xerographic copier. Alternatively, the surface (22) may be a microencapsulated medium as disclosed in U.S. Patent Nos. 4,440,846 and 4,399,209. In such a medium, the imaging sheet comprises a support substrate for the layer of microcapsules on its surface. The microcapsules contain an internal phase of a photocurable photosensitive composition and a color former. The microcapsules are imagewise exposed to actinic radiation and broken so that the color former reacts with the developer substance and forms an image.

U.S. Patent No. 4,399,209 discloses a photosensitive medium as a transfer imaging system. The imaging sheet is brought into contact with a receiver sheet after exposure. The receiver sheet comprises a developer sheet having a carrier of a layer of developer on its surface. After the sheets are brought into contact, the microcapsules are broken. U.S. Patent No. 4,440,846 discloses a self-contained imaging system in which developer and microcapsules are simultaneously supported on the surface of the imaging sheet.

The focusing system (20) may be any of the systems known in the art that are typically used in scanning slit-type copiers. For example, a Selfoc lens arrangement may be used. Alternatively, a spherical lens may be used.

It should also be noted that, although in the specific embodiment described here the original document remains stationary while the light sources and the focusing system are moved for scanning, these latter parts may also remain stationary while the original document and the photosensitive surface are moved for scanning. Both techniques are well known in the art of copying machines.

In the present device, the reduction of contrast is achieved by using a burst exposure. The extent of this burst exposure can be adjusted to effect contrast control. The burst exposure is achieved by covering a portion of the field of view of the focusing system (20) with a diffuse light reflector (28). Such a reflector can be any suitable diffusely reflecting surface, such as a metal surface that has been painted white. The extent of the Burst exposure is regulated by positional alignment of the plate (28) in the directions indicated by the arrows (30). A portion of the illumination light from the lamps (16) strikes the plate (28) and is diffusely reflected. A portion of the reflected light is received by the focusing system (20) and directed onto the photosensitive surface (22). By aligning the portion of the field of view of the original document (14) that is covered by the plate (28), the extent of the burst exposure is regulated. Burst exposure can be gradually increased from up to 100% by moving the plate outside the view of the focusing system or completely covering the field of view of the focusing system.

While the plate (28) in Fig. 1 is shown slightly offset for clarity, it should preferably be positioned relatively close to the glass plate (12) and the document (14).

Any suitable simple adjustment mechanism may be used, depending on the design details of the copying machine and the particular exposure and focusing systems used. In general, once the relative position of the platen (28) with respect to the lamps (16) and the focusing system (20) is fixed, that relative position of the platen should remain unchanged. However, in some cases, movement of the platen (28) may be desirable, such as when copying an original document containing both text and graphic portions.

An arrangement for adjusting the position of the plate (28) is shown in Fig. 2. It is noted that the complete device shown in Fig. 2 is fixed in position relative to the exposure lamps and the focusing system. In the embodiment shown in Fig. 1, in which the scanning system (10) is in relation to the original document (14) moves, the adjustment device shown in Fig. 2 is mounted as part of the scanning system (10).

A wire (32) is attached which extends around a plurality of rollers (34) in the manner shown in Fig. 2. The plate (28) is secured to the wire (32) with a suitable fastener (36). The wire (32) and plate (28) are positioned over the lamps (16) (the reflectors (18) have been omitted in Fig. 2 for simplification) and the focusing system (20).

An adjustment lever (38) is connected to the wire (32) and can be moved in both directions and as defined by the wire (32). A guide plate (40) with a groove (42) through which the lever (38) extends is mounted over the wire (32). Movement of the lever (38) in a leftward direction as viewed in Fig. 2 results in movement of the plate (28) as shown by the arrows (44) and reduces the reflected burst exposure light and thus increases the contrast. The opposite movement of the lever (38) leads to opposite results. By corresponding movement of the lever (38), the extent to which the plate (28) covers the field of view of the focusing system (20) can be adjusted.

The expected effect of shock exposure on contrast control is shown in Fig. 3, where the optical density (D) of an image to be reproduced is plotted as a function of the logarithm of the exposure energy (log E). While similar curves are expected using other photosensitive media, the curves shown in Fig. 3 are intended to specifically illustrate the performance of the microencapsulated photosensitive medium. Curve 46 shows the performance of the medium without shock exposure. As can be seen, the high average slope between the values Dmin and Dmax is relatively steep. Curves (48), (50) and (52) illustrate increasing burst exposure levels and consequently decreasing contrast levels. As can be seen, as the amount of burst exposure increases, the average slope of the curves shown in Fig. 3 decreases.

An advantage of the apparatus disclosed here is that the burst exposure is provided by the same lamp as that which provides the actinic radiation for the imagewise exposure. Thus, if the light source and medium are properly matched, a corresponding match for the burst exposure is also provided. Such a match can be difficult if an auxiliary lamp is used to achieve a burst exposure.

With reference to Fig. 3, it is also noted that the peak exposure level at which Dmin is achieved is independent of the amount of burst exposure used to control contrast. Therefore, independent control of brightness can be provided to increase the flexibility of the control system.

As an alternative to white diffuse reflectors, colored plates (28) or a colored filter placed in front of the plate (28) may be used to affect the color content of the burst exposure. Such a feature may be desirable for better control of the quality of the output image, although it would increase the complexity of the device.

Finally, with reference to Fig. 3, it is also pointed out that while the device has an effect on the average slope of the curves shown, the shape of the curves changes in a predetermined manner and complete control over the shape of the curves is not possible. However, it is believed that the disadvantage of the lack of such a Control is offset by the low complexity, cost and energy requirements of the contrast control system disclosed here.

Claims (6)

1. Exposure device for original documents, comprising - a support to carry the original; - a slit illumination system for illuminating, in operation, a slit surface on the original document, which is carried by the support and which comprises a light source which, in operation, directs illumination light onto the carried original document, and further comprises a light directing device for illuminating, in operation, reflected illumination light onto a light-sensitive medium which the light directing device has received from the field of view of the original document visible to it, the field of view comprising the slit surface, - a movement device which, during operation, causes a relative movement between the support and the slit illumination system in order to work with the illumination light and the field of view to scan the original document - and a reflector, characterized by an adjustable reflector positioning device attached to the slit illumination system to position the reflector between the support and the slit illumination system to a position suitable to block a portion of the field of view and to adjust the relative proportion of the field of view with respect to the slit illumination system and to maintain such relative proportion during scanning of the original document with illuminating light until further adjustment is made, so that the reflector is adjusted in use to receive a portion of the illuminating light from the light source before it reaches the original document and so the reflector reflects the portion of the illuminating light diffusely to the field of view of the light directing device. 2. Device according to claim 1, characterized in that the adjustable reflector positioning device contains a selective movement device for moving the reflector selectively into and out of the field of view. 3. Device according to claim 2, characterized in that the support comprises a flat support insert for the original document, that the reflector defines a flat reflection surface which lies in a plane parallel to the flat support insert, and that the selective movement device is arranged to move the reflection surface in said parallel plane. 4. Device according to claim 3, characterized in that the reflector is a flat reflector plate. 5. Device according to one of claims 2 - 4, characterized in that the selective movement device is designed to move the reflection surface perpendicular to the gap. 6. Device according to claim 4, characterized in that it contains a device for selectively moving the reflector plate perpendicular to the gap.